Inventing new tech and techniques to make his movies the way he wants to make them is James Cameron’s thing now. For Avatar he came up with an entire 3D camera system that’s now being used regularly by other directors, and for the sequels, he’s planning to outdo himself tenfold. A lot of the next few movies takes place under the water, and Cameron and his crew invented a new system of motion-capture that can be used in submarine environments. But filming motion-capture in a big pool is way harder than you might think.

Cameron recently explained to Collider how he went about coming up with a way to shoot his films underwater, and the unexpected wrinkles they ran into when it came to capturing images in a camera while fighting the light-bending properties of water.

Well, we’re doing it. It’s never been done before and it’s very tricky because our motion capture system, like most motion capture systems, is what they call optical base, meaning that it uses markers that are photographed with hundreds of cameras. The problem with water is not the underwater part, but the interface between the air and the water, which forms a moving mirror. That moving mirror reflects all the dots and markers, and it creates a bunch of false markers. It’s a little bit like a fighter plane dumping a bunch of chaff to confuse the radar system of a missile. It creates thousands of false targets, so we’ve had to figure out how to get around that problem, which we did. Basically, whenever you add water to any problem, it just gets ten times harder. So, we’ve thrown a lot of horsepower, innovation, imagination and new technology at the problem, and it’s taken us about a year and a half now to work out how we’re going to do it.

He also described how he and his crew had systematically turned their young cast of unknowns almost literally into fish-people, training them to hold their breaths for minutes at a time underwater so that they can act like they’re saying lines.

We’ve done a tremendous amount of testing, and we did it successfully, for the first time, just last Tuesday [November 14th]. We actually played an entire scene underwater with our young cast. We’ve got six teenagers and one seven-year-old, and they’re all playing a scene underwater. We’ve been training them for six months now, with how to hold their breath, and they’re all up in the two to four minute range. They’re all perfectly capable of acting underwater, very calmly while holding their breath. We’re not doing any of this on scuba. And we’re getting really good data, beautiful character motion and great facial performance capture. We’ve basically cracked the code.

At this point, the plots of all these movies is secondary to the fact that I just want to see what Cameron’s going to do with all this weird stuff. If he’s going to all this trouble, the end result had better be something really spectacular.

Avatar 2 will hit theaters on December 18, 2020, Avatar 3 on December 17, 2021, Avatar 4 on December 20, 2024; and Avatar 5 on December 19, 2025.